This is dirs_links/t_unlink.c (Listing 18-1, page 347), an example from the book, The Linux Programming Interface.

The source code file is copyright 2022, Michael Kerrisk, and is licensed under the GNU General Public License, version 3.

This page shows the "distribution" or "book" version of the file (why are there two versions?), or the differences between the two versions. You can switch between the views using the tabs below.

In the listing below, the names of Linux system calls and C library functions are hyperlinked to manual pages from the Linux man-pages project, and the names of functions implemented in the book are hyperlinked to the implementations of those functions.

  Cover of The Linux Programming Interface
+/* t_unlink.c
+    Demonstrate that, when a file is unlinked, it is not actually removed from
+    the file system until after any open descriptors referring to it are closed.
+    Usage: t_unlink file
 #include <sys/stat.h>
 #include <fcntl.h>
 #include "tlpi_hdr.h"
 #define CMD_SIZE 200
 #define BUF_SIZE 1024
 main(int argc, char *argv[])
     int fd, j, numBlocks;
     char shellCmd[CMD_SIZE];            /* Command to be passed to system() */
     char buf[BUF_SIZE];                 /* Random bytes to write to file */
     if (argc < 2 || strcmp(argv[1], "--help") == 0)
         usageErr("%s temp-file [num-1kB-blocks] \n", argv[0]);
     numBlocks = (argc > 2) ? getInt(argv[2], GN_GT_0, "num-1kB-blocks")
                            : 100000;
+    /* O_EXCL so that we ensure we create a new file */
     fd = open(argv[1], O_WRONLY | O_CREAT | O_EXCL, S_IRUSR | S_IWUSR);
     if (fd == -1)
     if (unlink(argv[1]) == -1)          /* Remove filename */
     for (j = 0; j < numBlocks; j++)     /* Write lots of junk to file */
         if (write(fd, buf, BUF_SIZE) != BUF_SIZE)
             fatal("partial/failed write");
     snprintf(shellCmd, CMD_SIZE, "df -k `dirname %s`", argv[1]);
     system(shellCmd);                   /* View space used in file system */
     if (close(fd) == -1)                /* File is now destroyed */
     printf("********** Closed file descriptor\n");
+    /* See the erratum for page 348 at http://man7.org/tlpi/errata/.
+       Depending on factors such as random scheduler decisions and the
+       size of the file created, the 'df' command executed by the second
+       system() call below does may not show a change in the amount
+       of disk space consumed, because the blocks of the closed file
+       have not yet been freed by the kernel. If this is the case,
+       then inserting a sleep(1) call here should be sufficient to
+       ensure that the the file blocks have been freed by the time
+       of the second 'df' command.
+    */
     system(shellCmd);                   /* Review space used in file system */

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